High strength thin steel excellent in hole expansibility, ductility and chemical treatment characteristics, and method for production thereof

ABSTRACT

High strength hot rolled steel sheet having at least a 590 N/mm 2  tensile strength and excellent in elongation and ability of phosphate coating, that is, high strength hot rolled steel sheet excellent in burring, elongation, and ability of phosphate coating having a tensile strength of 590 N/mm 2  or more comprising a steel composition containing, by mass %, C: 0.02 to 0.08%, Si: 0.50% or less, Mn: 0.50 to 3.50%, P: 0.03% or less, S: 0.01% or less, Al: 0.15 to 2.0%, and the balance of iron and unavoidable impurities, satisfying Mn+0.5×Al&lt;4, having a microstructure of the steel sheet having a ratio of ferrite having a grain size of 2 μm or more of 40% or more.

TECHNICAL FIELD

The present invention high strength hot rolled steel sheet excellent inburring, elongation, and ability of phosphate coating used mainly forpress worked automotive chassis parts, having a thickness of 0.6 to 6.0mm or so, and having a strength of 590 N/mm² or more and a method ofproduction of the same.

BACKGROUND ART

In recent years, car bodies have been made lighter in weight as meansfor improving the fuel efficiency due to the environmental problemsraised by automobiles and a strong need has arisen for reducing costs byforming parts integrally and streamlining the working processes. Highstrength hot rolled steel sheet excellent in press workability hastherefore been developed. In the past, as such high strength hot rolledsteel sheet having a high workability, steel with a mixed structure of aferrite and martensite structure or ferrite and bainite structure orsteel with a substantially single phase structure of mainly bainite orferrite have been widely known.

In particular, steel of a ferrite and martensite structure has thecharacteristics of a high ductility and excellent fatiguecharacteristics, so is being used for automobile wheels etc. Forexample, Japanese Unexamined Patent Publication (Kokai) No. 6-33140discloses steel of a ferrite and martensite structure where the amountsof addition of Al and N in the ferrite and martensite structure areadjusted so as to leave solid solution N and obtain a high ageinghardening and thereby obtain a high fatigue strength, but in a ferriteand martensite structure, microvoids form around the martensite from thebeginning of deformation and lead to cracking, so there is the problemof poor burring. This made the steel unsuitable for applications such aschassis parts demanding a high burring.

Further, Japanese Unexamined Patent Publication (Kokai) No. 4-88125 andJapanese Unexamined Patent Publication (Kokai) No. 3-180426 disclosesteel sheet having a structure mainly comprised of bainite, but sincethe structure is mainly comprised of bainite, while the burring isexcellent, there is little of the soft ferrite phase, so the ductilityis poor. Further, Japanese Unexamined Patent Publication (Kokai) No.6-172924 and Japanese Unexamined Patent Publication (Kokai) No. 7-11382disclose steel sheet having a structure mainly comprised of ferrite, butsimilarly while the burring is excellent, hard carbides are made toprecipitate in order to secure strength, so the ductility is poor.

Further, Japanese Unexamined Patent Publication (Kokai) No. 6-200351discloses steel sheet excellent in burring and ductility having aferrite and bainite structure, while Japanese Unexamined PatentPublication (Kokai) No. 6-293910 discloses a method of production ofsteel sheet achieving both burring and ductility by use of two-stagecooling to control the ratio of ferrite. However, due to the furtherreduction in weight, complexity of parts, etc. of automobiles, furtherhigher burring and ductility are sought. Recent high strength, hotrolled steel sheets are being pressed to provide an advance level ofworkability not able to be handled by the above technology.

Further, Japanese Unexamined Patent Publication (Kokai) No. 2002-180190discloses an invention relating to high strength hot rolled steel sheetexcellent in burring and ductility. While high strength hot rolled steelsheet excellent in the contradictory characteristics of burring andductility has been obtained, in the hot rolling process, surface defectsknown as Si scale sometimes occurred resulting in damage to theappearance of the product. Further, high strength hot rolled steel sheetfor chassis parts etc. usually is chemically converted and painted afterpress working. However, problems sometimes arose such as cases of poorformation of the chemical conversion coating (poor chemical conversion)or cases of poor adhesion of the paint after application. These problemsare believed to be due to the large amount of Si contained in the steel.In this way, Si is often used for high strength hot rolled steel sheet,but various types of trouble arise.

Further, Japanese Unexamined Patent Publication (Kokai) No. 6-128688discloses technology for adjusting the hardness of the ferrite phase ina ferrite and martensite structure so as to improve the durability andachieve both ductility and fatigue strength. Further, JapaneseUnexamined Patent Publication (Kokai) No. 2000-319756 disclosestechnology for adding Cu to a ferrite and martensite structure so as tostrikingly improve the fatigue characteristics while maintaining theductility. In both cases, however, to secure sufficient ferrite in thehot rolling process, the amount of Si added becomes high, so in the hotrolling process, surface defects known as Si scale are formed in somecases and the appearance of the product is damaged in some cases.Further, high strength hot rolled steel sheet for chassis parts etc.normally is chemically converted and painted after press working.However, problems sometimes arose such as cases of poor formation of thechemical conversion coating (poor chemical conversion) or cases of pooradhesion of the paint after application.

DISCLOSURE OF THE INVENTION

The present invention was made so as to solve the above conventionalproblems and provides high strength hot rolled steel sheet excellent inelongation and remarkably improved in ability of phosphate coating bypreventing the drop in elongation accompanying an increase of strengthto a tensile strength of 590 N/mm² or more and further by preventing theformation of Si scale. That is, the present invention has as its objectto provide high strength hot rolled steel sheet excellent in burring,elongation, and ability of phosphate coating and a method of productionof that steel sheet. Its gist is as follows:

(1) High strength hot rolled steel sheet excellent in burring,elongation, and ability of phosphate coating characterized by being asteel composition containing, by mass %, C: 0.02 to 0.08%, Si: 0.50% orless, Mn: 0.50 to 3.50%, P: 0.03% or less, S: 0.01% or less, Al: 0.15 to2.0%, and the balance of iron and unavoidable impurities, satisfying thefollowing formula, having a microstructure of said steel sheet having aratio of ferrite of a grain size of 2 μm or more of at least 40%, andhaving a tensile strength of at least 590 N/mm²:Mn+0.5×Al<4  (1)

(2) High strength hot rolled steel sheet excellent in burring,elongation, and ability of phosphate coating characterized by having atensile strength of at least 590 N/mm² as set forth in (1), furthercontaining, by mass %, one or two or more of Ti: 0.003% to 0.20%, Nb:0.003% to 0.04%, V: 0.003% to 0.20%, Ca: 0.0005 to 0.01%, Zr: 0.0005 to0.01%, a REM: 0.0005 to 0.05%, and Mg: 0.0005 to 0.01%.

(3) High strength hot rolled steel sheet excellent in burring,elongation, and ability of phosphate coating characterized by having atensile strength of at least 590 N/mm² as set forth in (1) or (2),characterized by satisfying 0.3×Al+Si−2×Mn≧−4 . . . (2) and having amicrostructure of a grain size 2 μm or more ferrite and martensitetwo-phase structure.

(4) High strength, hot rolled steel sheet excellent in burring,elongation and ability of phosphate coating characterized by having atensile strength of at least 590 N/mm² as set forth in (1) or (2),characterized by having a microstructure of a grain size 2 μm or moreferrite and bainite two-phase structure.

(5) A method of production of high strength hot rolled steel sheetexcellent in burring, elongation, and ability of phosphate coatingcharacterized by having a tensile strength of 590 N/mm² or morecharacterized by ending hot rolling of a slab comprised of a steelcomposition as set forth in any one of (1) to (3) at a rolling endtemperature of the Ar₃ point or more, then cooling it by a cooling rateof 20° C./sec or more until 650° C. to 750° C., then air cooling it for2 to 15 seconds, further cooling it, then coiling it at a temperature ofless than 300° C.

(6) A method of production of high strength hot rolled steel sheetexcellent in burring, elongation, and ability of phosphate coatingcharacterized by having a tensile strength of 590 N/mm² or more,characterized by ending hot rolling of a slab comprised of a steelcomposition as set forth in any one of (1), (2), and (4) at a rollingend temperature of the Ar₃ point or more, then cooling it by a coolingrate of 20° C./sec or more to 650 to 800° C., then air cooling it for 2to 15 seconds, then further cooling it by a cooling rate of 20° C./secor more to 350 to 600° C. and coiling it.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a view of the relationship between Al and Mn and ability ofphosphate coating.

FIG. 2 is a view of the relationship between the 2 μm or larger ferritepercentage and the elongation.

FIG. 3 is a view of the relationship between elongation and strength.

BEST MODE FOR WORKING THE INVENTION

In conventional ferrite and martensite steel, securing ductilityrequires that a sufficient ferrite structure percentage be secured. Ahigh amount of addition of Si was essential. However, if the amount ofaddition of Si becomes high, surface defects known as Si scale areformed in some cases. It is known that these damage the appearance ofthe product and cause deterioration of the ability of phosphate coating.The inventors engaged in intensive studies to solve these problems andas a result discovered that to obtain a sufficient ferrite percentage inferrite and martensite steel, addition of Al is effective. They learnedthat by adjusting the Mn and the Al and Si ingredients and making theferrite grains at least a certain size as much as possible, even with alow amount of Si added, sufficient burring and elongation are obtained.Further, they discovered that by adjusting the Al and Mn, deteriorationof the ability of phosphate coating can be suppressed. By this, theinventors completed the present invention. That is, the inventors newlydiscovered that by making the specific microstructure of the steel sheeta low C-low Si-high Al system with Mn and Al and Si in a specificrelationship, high strength hot rolled steel sheet achieving highburring, elongation, and ability of phosphate coating can be obtained.Further, the inventors discovered an industrially advantageous method ofproduction for this.

Further, the present invention takes note of steel with a substantiallytwo-phase structure of ferrite and bainite where the ferrite improvesthe elongation and precipitates comprised of TiC, NbC, and VC secure thestrength and causes sufficient growth of the ferrite grains to improvethe elongation without lowering the burring, then causes the formationof precipitates to secure the strength so as to thereby solve the aboveproblems. That is, the inventors newly discovered that by obtaining aspecific microstructure of the present invention steel sheet comprisinga low C-low Si-high Al(Ti, Nb, V) system and having Mn and Al in aspecific relationship, high strength hot rolled steel sheetsimultaneously satisfying the three characteristics of burring,elongation, and ability of phosphate coating is obtained. Further, theydiscovered an industrially advantageous method of production for thesame. Note that (Ti, Nb, V) means inclusion of a specific amount of oneor more of Ti, Nb, and V.

Below, the reasons for limitation of the elements of the steelcomposition will be explained.

C is included in an amount of 0.02% to 0.08%. C is an element necessaryfor strengthening the martensite phase and securing strength. If lessthan 0.02%, the desired strength is hard to secure. On the other hand,if over 0.08%, the drop in the elongation becomes great, so the amountis made 0.02% to 0.08%.

Si is an important element for suppressing the formation of harmfulcarbides and obtaining a complex structure of mainly a ferrite structureplus residual martensite, but causes a deterioration of the ability ofphosphate coating and also forms Si scale, so 0.5% is made the upperlimit. If over 0.25%, at the time of production of hot rolled steelsheet, the temperature control for obtaining the above microstructuresometimes is severe, so the Si content is more preferably 0.25% or less.

Mn is an element necessary for securing strength. Therefore, 0.50% ormore must be added. However, if added in a large amount over 3.5%, microsegregation and macro segregation easily occur, the burring isdeteriorated, and a deterioration in the ability of phosphate coating isalso seen, to secure ability of phosphate coating without causingdeterioration of the elongation, the range of Mn must be 0.50% to 3.50%.

P dissolves in the ferrite and causes the elongation to drop, so itscontent is made 0.03% or less. Further, S forms MnS which acts as astarting point for breakage and remarkably lowers the burring andelongation, so the content is made 0.01% or less.

Al is one of the important elements in the present invention and isnecessary for achieving both elongation and ability of phosphatecoating. Therefore, 0.15% or more must be added. Al was an elementconventionally considered necessary for deoxidation in hot rolled steelsheet and normally was added in an amount of 0.01 to 0.07% or so. Theinventors ran various experiments on high strength hot rolled steelsheets based on steel compositions of low C-low Si systems includingremarkably large amounts of Al and different in metal structure andthereby reached the present invention. That is, they discovered that byincluding Al in an amount of 0.15% or more and forming the abovemicrostructure, it is possible to greatly improve the elongation withoutdamaging the ability of phosphate coating. With an amount of Al of 2.0%,the effect of improvement of the elongation becomes saturated. Not onlythis, but if added in an amount over 2.0%, achievement of bothelongation and ability of phosphate coating conversely ends up becomingdifficult, so the content is made 0.15% to 2.0%.

For achievement of both elongation and ability of phosphate coating, itis also important to define the relationship between Mn and Al. Whilethe reason is unclear, the inventors newly discovered that underconditions of Si of 0.5% or less, as shown in FIG. 1, under conditionsofMn+0.5×Al<4  (1)the ability of phosphate coating is not damaged.

Hot rolled steel sheet has to finish being controlled in microstructurein the extremely short time of ROT cooling. Up until now, themicrostructure was controlled during cooling by increasing the amount ofaddition of Si, but if the amount of addition of Si increases, there isthe problem that deterioration of the ability of phosphate coating isinduced. Deterioration of the elongation of types of steel requiringability of phosphate coating was unavoidable. Therefore, the inventorsengaged in intensive studies on techniques for improving the ability ofphosphate coating without causing the elongation to deteriorate andnewly discovered Al as an element which, like Si, forms ferrite and yetdoes not induce deterioration of the ability of phosphate coating andfurther does not cause deterioration of other aspects of quality.Further, the inventors engaged in repeated studies on the control of themicrostructure in a short time in addition of low Si-high Al, which wasnot clear up to now, and discovered that particularly in the low Si-highAl region in the region of addition of a high amount of Al of 0.15% ormore, control of the microstructure in a short time is difficult unlessconsidering the addition of Si, Al, and Mn. By clarifying theirindividual effects, the inventors arrived at the right side of formula(2). When this value is −4 or more, even with short treatment such ashot rolling ROT, a sufficient ferrite phase can be secured and a highelongation can be obtained. On the other hand, when this value is lessthan −4, the ferrite phase insufficiently grows and deterioration of theelongation is induced. From this, the inventors obtained the conditionof formula (2).0.3×Al+Si−2×Mn≧−4  (2)

Ti, Nb, and V cause the precipitation of fine carbides such as TiC, NbC,and VC and enable higher strength. For this purpose, it is necessary toadd one or more of Ti in an amount of 0.003 to 0.20%, Nb in an amount of0.003% to 0.04%, and V in an amount of 0.003% to 0.20%. With an amountof Ti, Nb, or V of less than 0.003%, it is difficult to obtain a rise instrength through precipitation strengthening, while if Ti exceeds 0.20%,Nb exceeds 0.04%, or V exceeds 0.20%, too large an amount of precipitateis formed and the elongation deteriorates. Further, for furthereffective use of precipitates of Ti, Nb, and V, Ti is preferablycontained in an amount of 0.020% or more, Nb in an amount of 0.010% ormore, and V in an amount of 0.030% or more.

Ca, Zr, and REMs are elements effective for controlling the morphologyof sulfide-based inclusions and improving the burring. To make theireffects of control of the morphology more effective, it is preferable toadd one or more of Ca, Zr, and a REM in an amount of at least 0.0005%.On the other hand, addition of large amounts induces coarsening of thesulfide-based inclusions and causes deterioration of the cleanliness.Even in low C-low Si-high Al ingredient system of the present invention,not only is the elongation lowered, but also a rise in the cost isinduced, so the upper limit of Ca and Zr is made 0.01% and the upperlimit of a REM is made 0.05%. Further, as a REM, for example, there arethe elements of the Element Nos. 21, 39, and 57 to 71.

As unavoidable impurities, even if containing for example N≦0.01%,Cu≦0.3%, Ni≦0.3%, Cr≦0.3%, Mo≦0.3%, Co≦0.05%, Zn≦0.05%, Na≦0.02%,K≦0.02%, and B≦0.0005%, the present invention is not exceeded.

The size of the ferrite grains is one of the most important indicatorsin the present invention. The inventors engaged in intensive researchand as a result discovered that if the area ratio of ferrite having agrain size of 2 μm or more is 40% or more, the result is steel sheetexcellent in elongation. FIG. 2 shows the relationship between the ratioof ferrite having a grain size of 2 μm or more and the elongation. Thisshows that if the ratio of ferrite having a grain size of 2 μm or moreis 40% or more, the steel sheet exhibits a high elongation.

This is believed to be because if the grain size is less than 2 am, theindividual crystal grains will not sufficiently recover and grow andwill therefore cause a drop in the elongation. Therefore, to achieveboth good burring and elongation, it is necessary to make the ratio offerrite having a grain size of 2 μm or more 40% or more. Note that toobtain a more remarkable effect, the ratio of ferrite having a grainsize of 3 μm or more being 40% or more is preferable. Further, the grainsize can be found by converting the area of the individual grains tocircle equivalent diameters.

The microstructure of the high strength hot rolled steel sheet is to becomprised of ferrite and martensite. Here, since the microstructurecontains ferrite with a grain size of 2 μm or more in an amount of 40%or more, the microstructure becomes a ferrite and martensite two-phasestructure with ferrite in an amount of 40% or more. For example, as themicrostructure of the present invention, one comprised of 40% or more offerrite of a grain size of 2 μm or more and the balance of ferrite witha grain size of less than 2 μm and martensite or one comprised of 40% ormore of ferrite of a grain size of 2 μm or more and the balance of onlymartensite may be used. The martensite is made 60% or less in this waybecause if the amount of martensite becomes greater than that, the dropin elongation becomes remarkably large. However, even if residualaustenite is contained in an amount of about 1% as measured by usualX-ray diffraction intensity, the ferrite and martensite two-phasestructure of the present invention is not exceeded. Further, even if theregion near the surface of the hot rolled steel sheet has a partialregion of extremely thin (for example, about 0.1 to 0.3 mm or so) carbonor another steel ingredient somewhat low, while the microstructure maydiffer somewhat, so long as the majority of the hot rolled steel sheetin the thickness direction is comprised of a microstructure of saidferrite and martensite two-phase structure with ferrite of a grain sizeof 2 μm contained in an amount of 40% or more, the action and effect ofthe present invention will remain.

The present invention provides high strength hot rolled steel sheethaving said steel composition and microstructure and further anindustrially advantageous method of production of high strength hotrolled steel sheet for producing that steel sheet.

When producing high strength hot rolled steel sheet by hot rolling, withthe low C-low Si-high Al system of the present invention, the finishrolling end temperature preferably is made the Ar₃ point or more so asto suppress the drop in elongation due to the rolling of the ferriteregion. However, if the temperature is too high, the coarsening of themicrostructure will induce a drop in the strength and elongation in somecases, so the finish rolling end temperature is preferably 1050° C. orless. Whether or not to heat the slab should be suitably determined bythe rolling conditions of the steel sheet, while whether to bond the hotrolled steel sheet with the next hot rolled steel sheet or slab duringthe hot rolling for continuous rolling should be suitably selectedaccording to whether the microstructure of the present invention can beobtained. Further, the steel may be melted by a converter system or anelectric furnace system. It is sufficient that the melting give theabove steel composition. Further, hot metal pretreatment, refining,degasification, etc. for controlling the impurities etc. should besuitably selected.

Rapidly cooling the steel sheet right after the end of the finishrolling is important for securing the ferrite ratio. The cooling rate ispreferably 20° C./sec or more. This is because if less than 20° C./sec,pearlite, which causes a drop in strength and a drop in elongation, isformed. Further, at 250° C./sec, the effect of suppression of pearlitebecomes saturated, but even over 250° C./sec, the ferrite crystal grainsgrow and ferrite with a grain size of 2 μm or more can be secured in anamount of 40% or more of the microstructure. If over 600° C./sec, theeffect of growth of the ferrite crystal grains also becomes saturatedand conversely maintenance of the shape of the hot rolled steel sheetbecomes no longer easy under the present circumstances, so 600° C./secor less is preferable.

It is important to stop the rapid cooling of the steel sheet once andair-cool the sheet in order to cause ferrite to precipitate and increaseits ratio and improve the elongation. However, if the air cooling starttemperature is less than 650° C., pearlite harmful to the burring isformed early. On the other hand, if the air cooling start temperature isover 750° C., the formation of ferrite is slow and the effect ofair-cooling is hard to obtain. Not only that, pearlite easily formsduring the subsequent cooling. Therefore, this is not desirable.Therefore, the air cooling start temperature is preferably 650 to 750°C. Further, even if the air cooling time is over 15 seconds, not onlywill the effect of increase in ferrite become saturated, but also theformation of pearlite will cause a drop in the strength and elongation.Further, a load will be placed on the subsequent control of the coolingrate and coiling temperature, so this is industrially not preferable.Therefore, the air cooling time is made 15 seconds or less. Note thatwith an air cooling time of less than 2 seconds, the ferrite cannot bemade to sufficiently precipitate, so this is not preferable. Further,the air cooling of the present invention includes, to an extent nothaving an effect on the formation of the subsequent microstructure,blowing a small amount of a mist-like coolant for the purpose ofchanging the scale near the surface of the hot rolled steel sheet.

After the air cooling, the hot rolled steel sheet is again rapidlycooled. The cooling rate again has to be at least 20° C./sec. If lessthan 20° C./sec, harmful pearlite is easily formed, so this is notpreferable. The effect of formation of bainite substantially becomessaturated at 200° C./sec. Further, over 600° C., sometimes the steelsheet is partially overcooled and local fluctuations in hardness occur,so this is not preferable.

Further, the stopping temperature of this rapid cooling (secondary rapidcooling), that is, the coiling temperature, is made 300 to 600° C. Ifthe coiling temperature is less than 350° C., hard martensitedetrimental to the burring is formed. On the other hand, if over 600°C., pearlite detrimental to the burring is easily formed.

By combining the present steel composition and hot rolling conditions asexplained above, it is possible to produce high strength hot rolledsteel sheet excellent in burring, elongation, and ability of phosphatecoating having a tensile strength of 590 N/mm² or more, where themicrostructure of the steel sheet is a ferrite and martensite two-phasestructure having a percent of ferrite having a grain size of 2 μm ormore of 40% or more. Further, even if the steel sheet of the presentinvention is treated on its surface (for example, coated with zinc orlubricated), the effect of the present invention stands and the presentinvention is not exceeded.

EXAMPLE 1

Steels having the chemical compositions shown in Table 1-1 and Table 1-2(content in mass %, blank fields indicating none added) were melted inconverters and continuously cast into slabs which were then rolled underthe hot rolling conditions shown in Table 2 and cooled to therebyproduce hot rolled steel sheets of thicknesses of 2.6 (Examples 1 to 16and Comparative Examples 1 to 3) and 3.2 mm (Examples 17 to 32 andComparative Examples 4 to 6). Note that the rate of rapid cooling wasmade 40° C./sec (Examples 1 to 15 and Comparative Examples 1 to 4), 120°C./sec (Examples 16 to 30 and Comparative Example 5), and 300° C./sec(Examples 31 and 32 and Comparative Example 6), and the air cooling timewas made 10 seconds (Examples 1 to 32 and Comparative Examples 1 to 6).However, the finish rolling end temperature of the hot rolling was 900°C. (Examples 1 to 32 and Comparative Examples 4 to 9) and 930° C.(Comparative Examples 1 to 3).

The thus obtained hot rolled steel sheets were subjected to tensiletests and burring tests, were observed for microstructure, and wereevaluated for ability of phosphate coating. The results are shown inTable 2-1 and Table 2-2.

Note 1) Tensile Strength and Elongation

The test pieces were subjected to tensile tests using JIS No. 5 piecesbased on JIS Z 2201.

Note 2) Burring

The burring tests were conducted by widening a punched hole having aninitial hole diameter (d0: 10 mm) by a 60° conical punch and finding theburring value (λ value)=(d−d0)/d0×100 from the hole diameter (d) whenthe crack passed through the sheet thickness so as to evaluate theburring. The results are shown in Table 2.

Note 3) Microstructure of Steel Sheet

In observing the microstructure, the sheet was corroded by Nytal, then ascan type electron microscope was used to identify the ferrite andbainite. The area ratio of ferrite of a grain size of 2 μm or more wasmeasured by image analysis.

Note 4) Ability of Phosphate Coating

For the ability of phosphate coating of hot rolled steel sheet, thesurface scale was removed, then a phosphate coating solution SD5000(made by Nippon Paint) was used for test of phosphate coating after theprescribed degreasing and surface conditioning. The phosphate coatingwas judged by SEM (scanning electron microscopy) with uniformly formedcoatings judged as “G (good)” and partially formed coatings as “P(poor)”.

Examples 1 to 32 are examples of the present invention having all of thechemical ingredients, finish rolling end temperature, air cooling starttemperature, and coiling temperature in the scope of the presentinvention, having microstructures comprised of the two phases of ferriteand bainite, and having percents of ferrite having a grain size of 2 μmor more of 40% or more, i.e., are high strength hot rolled steel sheetexcellent in burring, elongation, and ability of phosphate coatinghaving high λ values and elongation. On the other hand, the sheets ofthe comparative examples of Comparative Examples 1 to 9 deviated fromthe conditions of the present invention are inferior in the balance ofstrength, burring, and elongation and in the ability of phosphatecoating.

Further, while not shown in Table 1 and Table 2, when using a slab ofthe steel ingredients shown in Example 1 and hot rolling it at a hotrolling end temperature of 920° C., then cooling it to 625° C. byprimary rapid cooling (cooling rate of 40° C./sec), air-cooling it by anair cooling start temperature of 625° C. for 10 seconds, and furthercooling it by secondary rapid cooling (cooling rate of 20° C./sec, toobtain a coiling temperature of 460° C., since the air cooling starttemperature was lower than the scope of the present invention, severalpercent of pearlite formed in the microstructure and the area ratio offerrite having a grain size of 2 μm or more was a low 36% or outside thescope of the present invention. Therefore, the elongation became 19% andthe λ value became 95%, so the balance of burring and elongation waspoor. Further, when similarly using a slab of the steel ingredientsshown in Example 1 and hot rolling it at a hot rolling end temperatureof 910° C., then cooling it to 675° C. by primary rapid cooling (coolingrate of 100° C./sec), air cooling it by an air cooling start temperatureof 680° C. for 10 seconds, then further cooling it by secondary rapidcooling (cooling rate of 20° C./sec) to obtain a coiling temperature of320° C., since the coiling temperature was lower than the scope of thepresent invention, 10% or so of martensite formed in the microstructureand the area ratio of ferrite having a grain size of 2 μm or more was alow 33%, so the elongation became 20%, the λ value became 63%, and againthe balance of the burring and elongation ended up becoming poor. TABLE1-1 Steel composition (mass %) Mn + 0.5 C Si Mn P S N Al Nb Ti V Ca ZrREM Mg Al Ex. 1 0.03 0.01 1.50 0.015 0.0100 0.0030 0.40 0.010 0.0200.050 1.70 Ex. 2 0.03 0.01 1.23 0.015 0.0100 0.0030 0.60 0.040 0.2000.050 1.53 Ex. 3 0.03 0.005 3.00 0.001 0.0020 0.0005 1.10 0.020 0.0600.100 3.55 Ex. 4 0.03 0.02 2.40 0.005 0.0050 0.0010 1.40 0.010 0.0500.0025 0.0025 3.10 Ex. 5 0.03 0.02 0.60 0.012 0.0060 0.0050 2.00 0.0000.150 0.100 0.0025 1.60 Ex. 6 0.04 0.30 1.60 0.030 0.0100 0.0030 0.400.020 0.060 0.0025 1.80 Ex. 7 0.05 0.01 2.50 0.040 0.0020 0.0100 0.500.010 0.040 0.0040 2.75 Ex. 8 0.04 0.01 1.56 0.030 0.0010 0.0080 0.800.040 0.030 0.060 0.0025 0.0060 1.96 Ex. 9 0.04 0.005 0.56 0.015 0.00100.0009 1.40 0.020 0.100 0.0010 1.26 Ex. 10 0.05 0.02 1.23 0.012 0.00150.0020 2.00 0.010 0.050 0.010 0.0080 0.0025 0.0350 2.23 Ex. 11 0.05 0.022.50 0.012 0.0020 0.0025 0.70 0.030 0.000 0.0060 0.0040 2.85 Ex. 12 0.050.015 1.00 0.015 0.0040 0.0035 0.60 0.020 0.020 0.070 0.0060 1.30 Ex. 130.07 0.20 0.70 0.020 0.0020 0.0040 0.80 0.010 0.040 0.020 1.10 Ex. 140.06 0.01 0.56 0.008 0.0100 0.0025 1.40 0.040 0.100 0.050 0.0320 1.26Ex. 15 0.06 0.02 1.80 0.012 0.0100 0.0020 1.70 0.050 0.0025 0.0100 2.65Ex. 16 0.06 0.02 1.56 0.012 0.0040 0.0025 0.40 0.010 0.030 0.030 0.00250.0040 0.0100 1.76 Ex. 17 0.08 0.015 0.60 0.015 0.0010 0.0035 0.50 0.0800.070 0.0010 0.0060 0.85 Ex. 18 0.08 0.01 3.50 0.016 0.0100 0.0040 0.800.020 0.040 0.020 0.0080 3.90 Ex. 19 0.08 0.01 3.00 0.008 0.0020 0.00251.40 0.010 0.230 0.050 0.0080 3.70 Ex. 20 0.08 0.005 1.56 0.002 0.00100.0015 2.00 0.040 0.150 0.030 2.56

TABLE 1-2 Steel composition (mass %) Mn + 0.5 C Si Mn P S N Al Nb Ti VCa Zr REM Mg Al Ex. 21 0.05 0.01 0.60 0.016 0.0010 0.0040 0.60 0.0100.100 0.020 0.0025 0.90 Ex. 22 0.06 0.01 0.80 0.008 0.0015 0.0025 0.800.040 0.000 0.050 0.0025 0.0025 1.20 Ex. 23 0.06 0.02 2.30 0.012 0.00200.0020 1.40 0.030 0.050 0.0010 0.0035 3.00 Ex. 24 0.06 0.02 1.56 0.0120.0040 0.0025 1.70 0.010 0.030 0.020 0.0080 2.41 Ex. 25 0.08 0.015 0.800.015 0.0100 0.0035 0.60 0.040 0.020 0.070 0.0020 0.0100 1.10 Ex. 260.04 0.01 3.20 0.016 0.0020 0.0040 1.20 0.040 0.200 0.150 0.0025 3.80Ex. 27 0.04 0.01 1.23 0.008 0.0010 0.0025 1.40 0.010 0.230 0.050 0.00401.93 Ex. 28 0.04 0.005 1.56 0.002 0.0010 0.0015 2.00 0.040 0.150 0.0300.0060 0.0300 2.56 Ex. 29 0.05 0.015 0.80 0.015 0.0015 0.0035 1.50 0.0200.060 0.030 1.55 Ex. 30 0.05 0.01 1.20 0.016 0.0020 0.0040 0.80 0.0400.020 0.070 0.0025 1.60 Ex. 31 0.05 0.01 2.50 0.008 0.0040 0.0025 1.400.040 0.040 0.020 0.0040 3.20 Ex. 32 0.08 0.005 1.56 0.002 0.0020 0.00152.00 0.010 0.230 0.050 0.0060 2.56 Comp. Ex. 1 0.005 0.01 3.00 0.0150.010 0.0030 3.00 0.020 0.050 0.010 0.0025 4.50 Comp. Ex. 2 0.010 1.503.20 0.015 0.010 0.0030 2.10 0.010 0.050 0.050 0.0040 4.25 Comp. Ex. 30.015 1.50 2.20 0.001 0.002 0.0005 0.04 0.040 0.050 0.100 0.0060 2.22Comp. Ex. 4 0.12 0.80 3.50 0.005 0.005 0.0010 1.20 0.020 0.100 0.00104.10 Comp. Ex. 5 0.20 1.20 2.50 0.012 0.012 0.0050 0.04 0.020 0.3000.0080 2.52 Comp. Ex. 6 0.15 0.60 2.50 0.015 0.010 0.0030 0.05 0.0100.400 0.050 0.0040 2.53 Comp. Ex. 7 0.12 0.80 3.50 0.005 0.005 0.00101.40 0.020 0.100 0.0010 4.20 Comp. Ex. 8 0.20 0.01 2.50 0.012 0.0120.0050 0.04 0.020 0.050 0.100 0.0080 2.52 Comp. Ex. 9 0.15 0.01 2.000.015 0.010 0.0030 0.05 0.010 0.100 0.050 0.0040 2.03Blank ingredient boxes indicate none added. Figures outside scope ofinvention are in italics.

TABLE 2-1 Air cooling Percent of start Coiling Tensile ferrite havingAbility of temperature temperature strength Elongation grain size of 2phosphate (° C.) (° C.) (N/mm²) (%) λ value μm or more (%) coatingRemarks Ex. 1 710 350 638 26 99 70 G Ex. 2 700 550 1,012 15 62 42 G Ex.3 720 600 963 19 66 54 G Ex. 4 650 450 692 28 94 82 G Ex. 5 680 420 82724 79 83 G Ex. 6 720 380 708 24 89 65 G Ex. 7 690 500 649 27 98 68 G Ex.8 710 520 725 24 88 66 G Ex. 9 700 550 664 28 98 84 G Ex. 10 720 480 61532 109 95 G Ex. 11 650 350 647 27 99 75 G Ex. 12 680 550 656 26 97 69 GEx. 13 720 600 580 30 111 84 G Ex. 14 690 450 777 24 83 74 G Ex. 15 710420 630 31 105 96 G Ex. 16 700 380 643 26 98 69 G Ex. 17 720 500 696 2491 63 G Ex. 18 650 350 843 22 76 59 G Ex. 19 710 550 1,173 15 55 51 GEx. 20 700 600 934 21 70 74 G

TABLE 2-2 Air cooling Percent of start Coiling Tensile ferrite havingAbility of temperature temperature strength Elongation grain size of 2phosphate (° C.) (° C.) (N/mm²) (%) λ value μm or more (%) coatingRemarks Ex. 21 720 450 648 26 98 71 G Ex. 22 650 420 618 28 104 79 G Ex.23 680 380 748 26 87 78 G Ex. 24 720 500 625 31 106 95 G Ex. 25 690 350701 24 91 67 G Ex. 26 680 350 1,363 12 47 44 G Ex. 27 720 600 992 18 6559 G Ex. 28 690 450 914 22 72 76 G Ex. 29 690 350 640 29 102 92 G Ex. 30680 550 718 24 89 66 G Ex. 31 720 600 787 24 82 72 G Ex. 32 690 4501,042 19 62 70 G Comp. Ex. 1 650 500 771 30 88 96 P Comp. Ex. 2 680 350944 23 69 94 P Comp. Ex. 3 720 550 1,019 15 61 45 P Comp. Ex. 4 690 6001,008 19 64 62 P Comp. Ex. 5 680 450 1,313 9 48 33 P Low duct. Comp. Ex.6 690 450 1,521 5 41 10 P Low duct. Comp. Ex. 7 690 600 1,008 20 64 66 PComp. Ex. 8 680 450 951 15 66 35 G Low duct. Comp. Ex. 9 690 450 889 1470 39 G Low duct.

EXAMPLE 2

Steels of the ingredients shown in Table 3-1 and Table 3-2 were meltedand cast into slabs by continuous casting in accordance with an ordinarymethod. Examples 33 to 58 show steels of ingredients in accordance withthe present invention, Comparative Example 10 shows steel with amountsof addition of C and P outside the scope of the present invention,Comparative Example 11 shows steel with an amount of addition of Mnoutside the scope, Comparative Example 12 shows steel with an amount ofaddition of Al outside the scope, Comparative Example 13 shows steelwith amounts of addition of Si and Al outside the scope, ComparativeExample 14 shows steel with amounts of addition of Si and Ti and Voutside the scope, Comparative Example 15 shows steel with amounts ofaddition of Si and Nb outside the scope, and Comparative Example 16shows steel with an amount of addition of Al outside the scope. Further,Comparative Example 10 shows steel with a formula (1) outside the scopeof the present invention, while Comparative Example 11 shows steel withformulas (1) and (2) outside the scope.

These steels were heated in heating furnaces at temperatures of 1200° C.or more and were hot rolled to obtain 2.6 to 3.2 mm thick hot rolledsteel sheets. The hot rolling conditions are shown in Table 4-1, Table4-2, and Table 4-3.

In Table 4-1, 33-4 shows an example where the cooling rate is low andoutside the scope of the present invention, 34-3 and 38-3 show aircooling start temperatures outside the scope of the present invention,and 37-3 and 39-3 show coiling temperatures outside the scope of thepresent invention. Further, 42-2 of Table 4-2 shows a shorter aircooling time.

The thus obtained hot rolled steel sheets were tested for tensilestrength and ability of phosphate coating. The TS, El, and phosphatecoating of the test pieces are shown in Table 4-1, Table 4-2, and Table4-3. FIG. 3 shows the relationship between strength and elongation. Itis learned that the invention steels are higher in elongation comparedwith the comparative steels and therefore superior.

Note that the test methods of tensile strength and elongation, themethod of measurement of the microstructure of the steel sheets, and themethod of judgment of ability of phosphate coating are the same inconditions as Example 1. TABLE 3-1 Steel composition (mass %) C Si Mn PS Al Nb Ti V Ca Zr REM Ex. 33 0.060 0.010 1.500 0.018 0.003 0.300 — — —— — — Ex. 34 0.055 0.300 1.220 0.011 0.002 0.250 — — — — — — Ex. 350.060 0.005 1.200 0.015 0.004 0.400 — 0.020 — 0.003 — 0.004 Ex. 36 0.0600.100 1.100 0.005 0.002 0.300 — — — — — — Ex. 37 0.060 0.010 1.220 0.0060.003 0.450 — — 0.180 — — — Ex. 38 0.065 0.010 1.220 0.006 0.003 1.000 —— — — — — Ex. 39 0.060 0.010 1.500 0.011 0.002 0.800 — — — 0.002 — — Ex.40 0.060 0.020 1.400 0.007 0.004 0.800 — 0.020 — — — — Ex. 41 0.0700.010 1.300 0.010 0.004 0.900 — 0.030 — 0.003 — — Ex. 42 0.080 0.0103.000 0.008 0.002 1.700 — — — — 0.001 — Ex. 43 0.080 0.400 2.000 0.0080.003 0.300 — — — — — — Ex. 44 0.075 0.020 0.600 0.012 0.009 0.400 0.035— — 0.003 — — Ex. 45 0.080 0.005 1.400 0.015 0.003 0.250 — 0.190 — — —0.005 Ex. 46 0.080 0.020 1.500 0.012 0.002 0.300 — 0.020 — — — — Ex. 470.080 0.010 1.400 0.011 0.003 0.350 — — — — — — Ex. 48 0.075 0.010 1.6000.006 0.004 0.350 0.020 — — — — — Ex. 49 0.080 0.010 1.600 0.015 0.0040.400 0.010 0.010 0.050 — — — Ex. 50 0.080 0.020 1.600 0.011 0.004 0.900— 0.025 — — 0.008 — Ex. 51 0.080 0.020 1.600 0.015 0.003 1.000 — — — — —— Ex. 52 0.080 0.005 1.400 0.015 0.003 1.400 — — — 0.003 — — Ex. 530.025 0.020 1.400 0.012 0.003 0.800 — — — — — 0.001 Ex. 54 0.050 0.0102.000 0.025 0.003 0.900 — — — — — 0.006 Ex. 55 0.050 0.020 2.200 0.0080.003 0.900 — — — — — — Ex. 56 0.060 0.010 2.000 0.017 0.003 0.900 — —0.010 — — — Ex. 57 0.060 0.250 2.200 0.017 0.003 0.200 — — — — — — Ex.58 0.060 0.350 2.400 0.016 0.003 0.250 — 0.025 — 0.003 — — Comp. Ex. 100.100 0.300 3.400 0.040 0.003 1.900 — — — — — — Comp. Ex. 11 0.060 0.2004.000 0.020 0.003 1.000 — — — — — — Comp. Ex. 12 0.060 0.100 1.500 0.0200.003 0.030 — — — — — — Comp. Ex. 13 0.055 0.700 1.500 0.020 0.004 2.500— — — — — — Comp. Ex. 14 0.056 0.800 1.100 0.020 0.010 0.200 — 0.2200.300 — — — Comp. Ex. 15 0.060 1.500 2.000 0.020 0.002 0.200 0.050 — — —— — Comp. Ex. 16 0.060 0.300 2.000 0.020 0.004 3.000 — — — — — —

TABLE 3-2 Equation 1, left Equation 2, right side side Ar₃ ° C. Ex. 331.65 −2.1 775 Ex. 34 1.35 −1.4 801 Ex. 35 1.40 −1.2 793 Ex. 36 1.25 −1.2799 Ex. 37 1.45 −1.1 790 Ex. 38 1.72 0.6 787 Ex. 39 1.90 −0.6 773 Ex. 401.80 −0.4 779 Ex. 41 1.75 0.1 780 Ex. 42 3.85 −0.9 667 Ex. 43 2.15 −2.7741 Ex. 44 0.80 0.0 823 Ex. 45 1.53 −2.0 770 Ex. 46 1.65 −2.1 763 Ex. 471.58 −1.7 769 Ex. 48 1.78 −2.1 758 Ex. 49 1.80 −2.0 757 Ex. 50 2.05 −0.5757 Ex. 51 2.10 −0.2 758 Ex. 52 2.10 1.4 770 Ex. 53 1.80 −0.4 798 Ex. 542.45 −1.3 750 Ex. 55 2.65 −1.7 733 Ex. 56 2.45 −1.3 743 Ex. 57 2.30 −3.6736 Ex. 58 2.53 −3.7 726 Comp. Ex. 10 4.25 −0.6 653 Comp. Ex. 11 4.50−4.8 621 Comp. Ex. 12 1.52 −2.8 777 Comp. Ex. 13 2.75 5.2 796 Comp. Ex.14 1.20 −0.8 824 Comp. Ex. 15 2.10 −1.9 783 Comp. Ex. 16 3.50 5.3 751* where, Ar₃ = 896 − 509(C %) + 26.9(Si %) − 63.5(Mn %) + 229(P %)

TABLE 4-1 Air cooling Air Percent of Ability of Finishing Cooling startcooling Coiling ferrite having Tensile phosphate temperature ratetemperature time temperature grain size of 2 strength Elongation coating° C. ° C./sec ° C. sec ° C. μm or more (%) N/mm² % % Ex. 33-1 920 70 6704 100 85 589 33 G Ex. 33-2 910 70 710 3 100 56 569 32 G Ex. 33-3 920 40660 3 100 73 599 32 G Ex. 33-4 930 10 750 5 100 72 589 22 G Ex. 34-1 92070 670 3 100 73 585 32 G Ex. 34-2 900 70 720 3 250 56 575 32 G Ex. 34-3910 70 780 2 100 20 590 24 G Ex. 34-4 890 40 680 2 100 55 590 31 G Ex.35-1 910 70 670 3 100 74 585 32 G Ex. 35-2 920 40 700 2 100 49 597 30 GEx. 36-1 890 70 670 4 100 89 571 34 G Ex. 36-2 930 70 650 3 250 81 55634 G Ex. 37-1 930 70 670 3 100 75 566 33 G Ex. 37-2 920 40 700 3 100 64576 32 G Ex. 37-3 920 70 720 3 350 57 551 22 G Ex. 38-1 910 70 680 3 10079 573 33 G Ex. 38-2 910 40 720 4 100 80 585 33 G Ex. 38-3 890 70 630 3100 92 573 26 G Ex. 39-1 920 70 680 3 100 74 607 32 G Ex. 39-2 920 70700 3 100 67 619 31 G Ex. 39-3 930 40 700 4 350 82 599 25 G Ex. 40-1 91070 690 3 100 71 608 31 G Ex. 40-2 900 40 730 4 100 72 620 31 G

TABLE 4-2 Air cooling Air Percent of Ability of Finishing Cooling startcooling Coiling ferrite having Tensile phosphate temperature ratetemperature time temperature grain size of 2 strength Elongation coating° C. ° C./sec ° C. sec ° C. μm or more (%) N/mm² % % Ex. 41-1 920 70 6803 100 77 623 31 G Ex. 41-2 910 40 700 3 100 70 635 30 G Ex. 42-1 880 70670 4 100 91 771 27 G Ex. 42-2 870 40 720 1 100 28 783 18 G Ex. 43-1 91070 670 4 100 82 724 28 G Ex. 43-2 890 70 680 4 250 78 709 28 G Ex. 44-1890 70 670 3 100 80 548 34 G Ex. 44-2 910 40 710 3 250 66 533 34 G Ex.45-1 890 70 670 3 100 70 955 19 G Ex. 45-2 890 50 680 3 100 66 955 18 GEx. 46-1 880 70 680 3 100 66 669 29 G Ex. 46-2 890 30 690 3 100 63 68128 G Ex. 47-1 920 70 670 3 100 71 611 31 G Ex. 47-2 910 70 690 3 100 64611 31 G Ex. 48-1 890 70 680 3 100 66 663 29 G Ex. 48-2 900 70 700 4 10074 663 30 G Ex. 49-1 900 70 670 4 100 85 665 30 G Ex. 49-2 890 150 660 3100 74 665 29 G Ex. 50-1 920 70 680 3 100 74 663 30 G Ex. 50-2 920 40690 3 100 71 675 29 G

TABLE 4-3 Air cooling Air Percent of Ability of Finishing Cooling startcooling Coiling ferrite having Tensile phosphate temperature ratetemperature time temperature grain size of 2 strength Elongation coating° C. ° C./sec ° C. sec ° C. μm or more (%) N/mm² % % Ex. 51-1 930 100660 4 100 98 630 32 G Ex. 51-2 910 70 720 3 100 62 630 30 G Ex. 52-1 90070 680 3 100 84 611 32 G Ex. 52-2 910 40 700 3 100 77 623 31 G Ex. 53-1890 70 680 4 100 90 525 36 G Ex. 53-2 890 40 700 3 100 68 537 34 G Ex.54-1 890 70 660 3 100 77 619 31 G Ex. 54-2 900 70 660 4 250 92 599 33 GEx. 55-1 920 70 700 3 100 61 644 29 G Ex. 55-2 930 70 660 3 250 75 62431 G Ex. 56-1 900 70 690 3 100 67 634 30 G Ex. 56-2 930 70 700 3 100 63639 30 G Ex. 57-1 890 70 680 4 100 74 670 29 G Ex. 57-2 910 70 690 3 25055 650 29 G Ex. 58-1 910 70 670 3 100 62 740 26 G Ex. 58-2 910 70 680 3250 58 715 27 G Comp. Ex. 10 850 70 710 3 100 38 836 16 P Comp. Ex. 11900 70 700 3 100 16 836 14 P Comp. Ex. 12 920 70 700 3 100 30 595 24 GComp. Ex. 13 900 70 720 2 100 74 618 31 P Comp. Ex. 14 900 70 680 3 10073 916 16 P Comp. Ex. 15 910 70 710 4 100 72 879 17 P Comp. Ex. 16 91070 710 3 100 93 643 31 P

INDUSTRIAL APPLICABILITY

As explained in detail above, according to the present invention, highstrength hot rolled steel sheet having a high strength of a tensilestrength of 590 N/mm² or more and excellent in burring, elongation, andability of phosphate coating can be economically provided, so thepresent invention is suitable as high strength hot rolled steel sheethaving a high workability. Further, the high strength hot rolled steelsheet of the present invention enables reduction of the weight of carbodies, integral formation of parts, and streamlining of the workingprocesses and therefore can contribute to the improvement of the fuelefficiency and reduction of production costs so is great in industrialvalue.

1. High strength hot rolled steel sheet excellent in burring,elongation, and ability of phosphate coating characterized by being asteel composition containing, by mass %, C: 0.02 to 0.08%, Si: 0.50% orless, Mn: 0.50 to 3.50%, P: 0.03% or less, S: 0.01% or less, Al: 0.15 to2.0%, and the balance of iron and unavoidable impurities, satisfying thefollowing formula, having a microstructure of said steel sheet having aratio of ferrite of a grain size of 2 μm or more of at least 40%, andhaving a tensile strength of at least 590 N/mm²:Mn+0.5×Al<4  (1)
 2. High strength hot rolled steel sheet excellent inburring, elongation, and ability of phosphate coating characterized byhaving a tensile strength of at least 590 N/mm² as set forth in claim 1,further containing, by mass %, one or two or more of Ti: 0.003% to0.20%, Nb: 0.003% to 0.04%, V: 0.003% to 0.20%, Ca: 0.0005 to 0.01%, Zr:0.0005 to 0.01%, a REM: 0.0005 to 0.05%, and Mg: 0.0005 to 0.01%. 3.High strength hot rolled steel sheet excellent in burring, elongation,and ability of phosphate coating characterized by having a tensilestrength of at least 590 N/mm² as set forth in claim 1 or 2,characterized by satisfying 0.3×Al+Si−2×Mn≧−4 . . . (2) and having amicrostructure of a grain size 2 μm or more ferrite and martensitetwo-phase structure.
 4. High strength, hot rolled steel sheet excellentin burring, elongation and ability of phosphate coating characterized byhaving a tensile strength of at least 590 N/mm² as set forth in claim 1or 2, characterized by having a microstructure of a grain size 2 μm ormore ferrite and bainite two-phase structure.
 5. A method of productionof high strength hot rolled steel sheet excellent in burring,elongation, and ability of phosphate coating characterized by having atensile strength of 590 N/mm² or more characterized by ending hotrolling of a slab comprised of a steel composition as set forth in claim1 or 2 at a rolling end temperature of the Ar₃ point or more, thencooling it by a cooling rate of 20° C./sec or more until 650° C. to 750°C., then air cooling it for 2 to 15 seconds, further cooling it, thencoiling it at a temperature of less than 300° C.
 6. A method ofproduction of high strength hot rolled steel sheet excellent in burring,elongation, and ability of phosphate coating characterized by having atensile strength of 590 N/mm² or more, characterized by ending hotrolling of a slab comprised of a steel composition as set forth in claim1 or 2 at a rolling end temperature of the Ar₃ point or more, thencooling it by a cooling rate of 20° C./sec or more to 650 to 800° C.,then air cooling it for 2 to 15 seconds, then further cooling it by acooling rate of 20° C./sec or more to 350 to 600° C. and coiling it.